Colour mixing in Blender Game Engine
As the RGB color space is 3Dimensional I suggest to directly set the object color via Python.

import bge

object = bge.logic.getCurrentController().owner

red = ... # value between 0.0 and 1.0
green = ... # value between 0.0 and 1.0
blue = ... # value between 0.0 and 1.0
alpha = 1.0

object.color = (red, green, blue, alpha)


You can calculate the "mixed" value by calculating the average:

def mean(*numbers):
    return sum(numbers)/len(numbers) if numbers else None


usage:

red = mean(0.3, 0.7)